Re: [PATCH v2] PM: hibernate: Fix a bug in copying the zero bitmap to safe pages

[Rafael J. Wysocki]

On Wed, Oct 4, 2023 at 7:01 AM Pavankumar Kondeti
<quic_pkondeti@xxxxxxxxxxx> wrote:
>
> The following crash is observed 100% of the time during resume from
> the hibernation on a x86 QEMU system.
>
> [   12.931887]  ? __die_body+0x1a/0x60
> [   12.932324]  ? page_fault_oops+0x156/0x420
> [   12.932824]  ? search_exception_tables+0x37/0x50
> [   12.933389]  ? fixup_exception+0x21/0x300
> [   12.933889]  ? exc_page_fault+0x69/0x150
> [   12.934371]  ? asm_exc_page_fault+0x26/0x30
> [   12.934869]  ? get_buffer.constprop.0+0xac/0x100
> [   12.935428]  snapshot_write_next+0x7c/0x9f0
> [   12.935929]  ? submit_bio_noacct_nocheck+0x2c2/0x370
> [   12.936530]  ? submit_bio_noacct+0x44/0x2c0
> [   12.937035]  ? hib_submit_io+0xa5/0x110
> [   12.937501]  load_image+0x83/0x1a0
> [   12.937919]  swsusp_read+0x17f/0x1d0
> [   12.938355]  ? create_basic_memory_bitmaps+0x1b7/0x240
> [   12.938967]  load_image_and_restore+0x45/0xc0
> [   12.939494]  software_resume+0x13c/0x180
> [   12.939994]  resume_store+0xa3/0x1d0
>
> The commit being fixed introduced a bug in copying the zero bitmap
> to safe pages. A temporary bitmap is allocated with PG_ANY flag in
> prepare_image() to make a copy of zero bitmap after the unsafe pages
> are marked. Freeing this temporary bitmap with PG_UNSAFE_KEEP later
> results in an inconsistent state of unsafe pages. Since free bit is
> left as is for this temporary bitmap after free, these pages are
> treated as unsafe pages when they are allocated again. This results
> in incorrect calculation of the number of pages pre-allocated for the
> image.
>
> nr_pages = (nr_zero_pages + nr_copy_pages) - nr_highmem - allocated_unsafe_pages;
>
> The allocate_unsafe_pages is estimated to be higher than the actual
> which results in running short of pages in safe_pages_list. Hence the
> crash is observed in get_buffer() due to NULL pointer access of
> safe_pages_list.
>
> Fix this issue by creating the temporary zero bitmap from safe pages
> (free bit not set) so that the corresponding free bits can be cleared while
> freeing this bitmap.
>
> Cc: stable <stable@xxxxxxxxxx>

Why is this tag present?  The commit below hasn't made it to a major
release yet AFAICS.

> Fixes: 005e8dddd497 ("PM: hibernate: don't store zero pages in the image file")
> Suggested-by:: Brian Geffon <bgeffon@xxxxxxxxxx>
> Signed-off-by: Pavankumar Kondeti <quic_pkondeti@xxxxxxxxxxx>
> ---
> Changes in v2:
> - Allocate zero bit map from safe pages as suggested by Brian
> - Link to v1: https://lore.kernel.org/r/20230929-hib_zero_bitmap_fix-v1-1-6cfdcb785250@xxxxxxxxxxx
> ---
>  kernel/power/snapshot.c | 4 ++--
>  1 file changed, 2 insertions(+), 2 deletions(-)
>
> diff --git a/kernel/power/snapshot.c b/kernel/power/snapshot.c
> index 87e9f7e2bdc0..0f12e0a97e43 100644
> --- a/kernel/power/snapshot.c
> +++ b/kernel/power/snapshot.c
> @@ -2647,7 +2647,7 @@ static int prepare_image(struct memory_bitmap *new_bm, struct memory_bitmap *bm,
>         memory_bm_free(bm, PG_UNSAFE_KEEP);
>
>         /* Make a copy of zero_bm so it can be created in safe pages */
> -       error = memory_bm_create(&tmp, GFP_ATOMIC, PG_ANY);
> +       error = memory_bm_create(&tmp, GFP_ATOMIC, PG_SAFE);
>         if (error)
>                 goto Free;
>
> @@ -2660,7 +2660,7 @@ static int prepare_image(struct memory_bitmap *new_bm, struct memory_bitmap *bm,
>                 goto Free;
>
>         duplicate_memory_bitmap(zero_bm, &tmp);
> -       memory_bm_free(&tmp, PG_UNSAFE_KEEP);
> +       memory_bm_free(&tmp, PG_UNSAFE_CLEAR);
>         /* At this point zero_bm is in safe pages and it can be used for restoring. */
>
>         if (nr_highmem > 0) {
>
> ---
> base-commit: 8a749fd1a8720d4619c91c8b6e7528c0a355c0aa
> change-id: 20230929-hib_zero_bitmap_fix-bc5884eba0ae
>
> Best regards,
> --
> Pavankumar Kondeti <quic_pkondeti@xxxxxxxxxxx>
>